An example of a prior art structure is a conventional angle sensor in which the sensing islands are four long anisotropic magneto-resistance (AMR) stripes 12 oriented in a diamond shape with the ends connected together by metallization to form a Wheatstone bridge, as shown in FIG. 1. Typically, these anisotropic magneto-resistance (AMR) stripes are made of Permalloy material. The top and bottom connections of the four identical islands are given a direct current stimulus in the form of a supply voltage (Vs), with the remaining side connections to be measured. In the absence of a magnetic field, the side contacts should be at the same voltage. To have the island magnetization direction align with an externally applied magnetic field, the latter must be large enough to saturate the Permalloy material.
With the AMR islands connected in this fashion to form the Wheatstone bridge, these side contacts will produce a different voltage (ΔV) which is a function of the supply voltage, the MR ratio, and the angle between the island current flow (I) and island magnetization (M). If there is only one such bridge, angle measurement is limited to a range of from −45 degree to +45 degree. When combined with a second bridge, which is rotated 45-degrees relative to the first bridge, a wider range of angle, from −90 to +90 degrees, can be measured.
In the prior art, due to the characteristics of the AMR effect in which the resistance change is a function of cos2(θ), where θ is the angle between the magnetization and current flowing direction, one AMR Wheatstone only detects 90-degree angle while two AMR Wheatstone bridges with 45-degrees orientation difference only allow a measurement of 180-degree angle. In order to measure a full 360-degree angle, an additional Hall sensor must be used in combination with the two Wheatstone bridges.
Due to the characteristics of GMR or MTJ devices, in which the resistance change is a function of cos(θ), where θ is the angle between the free layer magnetization and the pinned reference layer magnetization, they have the ability to detect the full 360-degree magnetic field. However, in an angle sensor using GMR or MTJ devices, it is required that the reference layers be pinned in various directions, thereby introducing a major challenge to GMR or MTJ based sensor development. In order to achieve maximum sensitivity and accuracy, the pinned magnetizations of the reference layers need to be in both anti-parallel and orthogonal directions as in, for example, an ideal arrangement of eight sensing islands 21 that have identical geometry, differing only in their pinned directions, as shown schematically in FIG. 2.
With continuing advances in micro-magnetic technology, both in regard to the structures formed and the processes needed to form them, the need arises for the ability to apply exchange pinning fields on two or more reference magnetic layers that share the same substrate, in different directions. Typically, an anti-ferromagnetic material (AFM) layer deposited directly underneath or on top of a soft ferromagnetic material layer is utilize to generate an exchange pinning field on the soft ferromagnetic layer through a thermal annealing process. The exchange pinning field direction is in the same direction of the thermal annealing field, i.e., the external field direction during the thermal annealing process. The problem with prior art approaches that uses GMR or MTJ devices has been that if, after applying an exchange pinning field on a first reference ferromagnetic layer in a first direction, it is attempted to apply another exchange pinning field on a second reference ferromagnetic layer in a different direction, application of the second thermal annealing field causes the direction of the first exchange pinning field on the first reference ferromagnetic layer to change.
The prior art approach to dealing with this problem has been to cut out individual sensing islands, all of which have been magnetized in the same direction on a single wafer, and to then rotate them through different angles, following which they are cemented in place and then connected together to form the completed angle sensor. Fabricating the latter in this fashion is expensive, limits the accuracy of the angle sensor, increases the size of the full structure, and is susceptible to the introduction of assembly errors.
A routine search of the prior art was performed with the following references of interest being found:
M. Ruhrig, et al., proposed a single-chip solution using pulsed electric currents to local conductor stripes to set magnetization directions of reference layers [3]. However, in this solution, one has to locally apply current pulse one by one to each GMR or MTJ island, which is costly and is not practical for a mass production.A. Jander, et al., U.S. Pat. No. 7,054,114 proposed a different solution without the need to apply local current pulses to set each individual GMR or MTJ island's pinning direction. In this prior art, four soft magnetic shields are arranged to have four orthogonal gaps regions where four active sensing islands are located. During thermal annealing process, as an external magnetic field is applied, the fields inside the shield gaps are magnified and their field directions altered to be essentially perpendicular to the gaps. As a result, the pinning directions of these four sensing islands are set perpendicular to gaps that they are located between them. However, using this approach, one still is not able to set pinning magnetizations with anti-parallel directions as required, for example, by an angle sensor based on a simple Wheatstone bridge.
In addition to the above references, the following publications were also found to be of interest:    1. Honeywell application note “Applications of Magnetic Position Sensors”    2. Taras Pokhil, et., “Exchange Anisotropy and Micromagnetic Properties of PtMn/NiFe bilayers,” J. Appi. Phys. 89, 6588 (2001)    3. M. Ruhrig, et al., “Angular Sensor Using Tunneling Magnetoresistive Junctions With An artificial Antiferromagnet Reference Electrode and Improved Thermal Stability,” IEEE Trans. Magn. V. 40, p. 101, January 2004